The present invention relates to an image forming apparatus with a shutter array and more particularly an improvement of an electrophotographic dot image formed by an electrophotographic printer with a liquid crystal shutter array light signal generating means.
The liquid crystal shutter array utilizes the electrooptical modulation capability of a liquid crystal. The liquid crystal modulation units are arranged in an array and light is illuminated on the modulation unit array in such a way that transmitted light rays are selectively derived, whereby a light signal corresponding to an electrical image signal can be obtained. The light signal is focused on an electrophotographic photosensitive body, whereby a digital copy can be obtained.
The advantages of such a crystal-optical shutter array are:
1. When used in an electrophotographic printer, the printer can be made compact in size and light in weight.
2. Mechanically movably parts such as a polygon scanner used in a laser beam printer can be eliminated so that no noise problem arises and strict mechanical tolerances can be reduced.
However, there is a disadvantage that an electrophotographic image formed by an electrophotographic printer with liquid crystal shutter array light signal generating means is inferior in resolution to an electrophotographic image formed by a laser beam printer. Especially in the case of a liquid crystal shutter array using a 1/2 times-sharing drive system type, there are provided two rows of shutter apertures and the shutter apertures in one row are staggered relative to those in the other row. In the first 1/2 period, the shutter apertures in the first row are operated and in the second 1/2 period, the shutter apertures in the second row are operated so that one line image may be formed. In this case, dot images formed by the operation of the shutter apertures in the first row are overlapped with dot images formed by the operation of the shutter apertures in the second row in one line image. As a result, one line image has density differences.
In the case of a liquid crystal shutter array using a 1/3 time-division drive system, there are provided three rows of shutter apertures and the shutter apertures in a first row are staggered relative to those in a second row which in turn are staggered relative to those in a third row. During a first 1/3 period, the shutter apertures in the first row are operated, during a second 1/3 period the shutter apertures in the second row are operated and during a third 1/3 period, the shutter apertures in the third row are operated so that one line image is formed. However, dot images formed by the operation of the shutter apertures in the first row are overlapped with the dot images formed by the operation of the shutter apertures in the second and third rows in one line image. As a result, one line image also has density differences. Furthermore, when one dot image is observed, the periphery of the dot image is "blurred" so that the dot image lacks sharpness.
In the case of a printer with a liquid crystal-optical shutter array of the type described above, a light signal which has been transmitted through an aperture cannot be sufficiently focused so that a dot image obtained has blurs and a low resolution.